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Interpretive Summary: Winter spreading of dairy manure is a pressing research and policy issue due to concerns about phosphorus (P) loss from fields in snowmelt runoff and water quality degradation. Policies controlling winter spreading are often based on anecdotal and not quantitative information. We used the SurPhos computer model to better quantify the impact of seasonal and short-term manure application on P loss. Results indicated that, for regions with significant winter runoff, winter manure application can substantially increase the risk of P loss. This is true for manure applied any time from late November through early March, with a maximum risk for application in late January and early February. Shifting manure application to fields with lower runoff potential and seasons of lower runoff likelihood can greatly reduce the risk of P loss. Delaying manure application when runoff is likely can also reduce P loss, but the number of opportunities producers may have to actually use application delays to reduce P loss is limited. Also, delaying application can sometimes increase P loss if manure is exposed to larger runoff events beyond the delay window. This study also showed that the SurPhos computer model does a good job of quantifying P loss for a range of conditions and management practices, thereby providing better manure management information for producers and policy makers.

Technical Abstract: Agricultural phosphorus (P) management is a pressing research and policy issue due to concerns about P loss from fields and water quality degradation. Better information is especially needed on the risk of P loss from dairy manure applied to fields in winter. We used the SurPhos computer model to assess the impact of seasonal and short-term manure application on dissolved P loss. For regions with significant winter runoff, winter manure application can substantially increase the risk of P loss, due to extended periods of high manure P availability and frequent and consistent runoff. The increased risk of P loss is true for manure applied any time from late November through early March, with a maximum risk for application in late January and early February. Shifting manure application to fields with lower runoff potential and seasons of lower runoff likelihood can greatly reduce the risk of manure P loss. Delaying manure application when runoff is imminent (near-term, tactical application decisions) can help reduce the risk of manure P loss any time of the year. However, the number of opportunities for producers to actively use application delays as a strategy to reduce P loss is limited. Also, delaying application can sometimes increase the risk of P loss if freshly applied manure is exposed to even larger runoff events beyond the delay window. Our study shows SurPhos can quantify P loss for a range of conditions and management practices and provide more robust manure management information for producers and policy makers.